The benefits of adding insulation to a structure are well known. Insulation restricts the flow of heat from a higher temperature (the "hot" side), through the insulating materials, to a lower temperature (the "cold" side). As soon as a temperature difference exists across an insulating material, thermal energy begins to flow. By slowing the rate of heat transfer, insulation enables conventional heating and cooling devices to maintain a desired temperature within a structure. However, because the heat begins to flow nearly instantaneously, a change in either the inside or the outside temperature is instantly reflected in a change in the rate of heat flow. In order to maintain the desired internal temperature, the heating and cooling equipment must be able to respond quickly to changes in the temperature difference. This may be difficult, however, because structures generally have either a large volume of air or a large mass in the internal environment, both of which resist rapid temperature changes. As a result, during rapid external temperature variations, the inside temperature is often either higher or lower than desired. It would be desirable to devise a method of maintaining a nearly constant rate of heat flow so as to maximize the efficiency of conventional heating and cooling equipment and to improve the correlation between the desired inside temperature and the actual inside temperature. In so doing, both the temperature variations and energy output required to maintain a desired inside temperature would be minimized.
Common forms of insulation include fiberglass batts and boards of extruded polystyrene. Other insulating materials include various vegetable and mineral fibers (generally used as blown-in insulation), other minerals (such as expanded perlite), and metal foils (for reflecting radiant thermal energy). Perlite is a naturally occurring volcanic glass which may be expanded to form an insulating material having many voids. It is the presence of such voids and the air or other gases that they entrap that provide the insulating properties of most insulating materials.
During the 1970s and 1980s, thermal research focused on development of suitable materials and configurations for storing solar energy during periods of peak solar insolation, and releasing it gradually during off-peak periods or during the night to the inside environment. These materials were generally built to store solar energy falling directly onto containers filled with phase change materials (hereinafter PCMs). PCMs attracted interest for their ability to store heat energy. PCMs are chemical compounds that use the latent heat properties of a material to store heat energy as chemical energy. "Latent heat" is a measure of the energy required to change a material from one state to another. A PCM stores heat by "melting" (or changing from a solid to a liquid) and releases heat by "freezing" (or changing from a liquid to a solid). Unfortunately, some PCMs are very hygroscopic, which means that they absorb water, which in turn decreases their effectiveness by providing a path for heat transfer through the PCM, in effect, a thermal short circuit.
The interest in phase change materials for building envelopes waned with the decrease in interest in active solar heating. The experience revealed, however, that the corrosive nature of the salts used as PCMs made it difficult to prevent PCM storage containers from leaking. Therefore it would be desirable to have these PCM materials stored in a medium that would not be affected by the corrosive salts, and that would seal the PCM off from atmospheric humidity.
The interest in solar energy developed in part from petroleum fuel shortages. The shortages and concomitant increased prices heightened both utilities' and consumers' awareness of their energy usage. This concern has remained to this day. Energy utility rates are generally lower at the time of day when the load on the utility system is the lightest. The utilities prefer to have a steady demand, and naturally, both commercial and private consumers prefer to pay lower rates. With ever increasing energy costs it would be desirable to move energy consumption to the time of day when the rates will be the lowest, thereby leveling out the load on the utility system as well.